Plastic film, particularly polyolefin film, has long been used in the packaging of fresh-cut flowers and vegetables. The primary function of such packaging film is to provide a protective seal against insects, bacteria and air-borne contaminants.
Nevertheless, fresh-cut flowers and vegetables have a limited shelf life due to fundamental changes in their biochemistry. Whilst in soil, photosynthesis allows the plant to grow and expire oxygen. However, after harvesting, the plant no longer converts sunlight and carbon dioxide into carbohydrates and oxygen, but uses its stores of carbohydrate to excrete water and carbon dioxide. For plants that are wrapped in plastic packaging, the accumulation of trapped water in the packaging leads to fungal growths and rotting of the fresh food. A study of volatiles produced by fruits and vegetables has been made by P.M.A. Toivonen (“Non-ethylene, non-respiratory volatiles in harvested fruits and vegetables; their occurrence, biological activity and control”, Postharvest Biology and Technology 12 (1997) 109-125). Plastic packaging film, particularly polyolefin film, acts as a water barrier and can actually make this situation worse by trapping excreted water within the package. Vegetables having a high moisture content, such as broccoli and mushrooms, are particularly susceptible. It would be desirable to provide packaging which not only provides a barrier to insects, bacteria and air-borne contaminants but which is also breathable to water (in other words, a film which would allow water to freely pass out of the packaging), thereby increasing the shelf life of the contents by delaying the onset of microbial and fungal growth.
There already exist a number of commercially available breathable films. Several companies market polyolefin films which have been voided using inorganic fillers thereby allowing easier passage of water. However, such films are not normally transparent and therefore have a disadvantage from a marketing standpoint. Other commercially available films based on thermoplastic elastomers (TPEs) provide excellent breathability but are expensive for packaging applications.
Another approach to the manufacture of breathable packaging has been to perforate the film in order to allow egress of excreted gas. The number of perforations per unit area and the perforation diameter may be adjusted depending on the goods to be packaged and the respiration rate thereof. The disadvantage of this approach is that the perforations expose the packaged goods to insects, bacteria and air-borne contaminants. Optionally heat-sealable composite films comprising a perforated layer are disclosed in WO-01/92000-A, GB-2355956-A, EP-0358461-A; EP-0178218-A; US-2002/0187694-A; JP-A-06/219465-A; JP-06/165636-A; and JP-54/117582-A. Co-pending application PCT/GB2004/003119 describes a breathable, heat-sealable, composite film comprising a perforated polymeric substrate layer and an unperforated barrier layer coated on a surface of the substrate.
It is an object of this invention to provide a breathable, heat-sealable packaging film which allows egress of moisture and excreted gas, particularly moisture, from the packaged product but which provides a physical barrier to insects, bacteria and airborne contaminants, particularly wherein the film is optically clear, and particularly wherein the manufacturing of the film can be achieved more efficiently and economically.